Photodecomposition of crystal violet dye in water solution and suspensions of metal oxides

Photodecomposition and photodecolorization of crystal violet dye were studied in water solution, hydrogen peroxide solution and suspensions of metal oxides. Spectroscopic evidence for the formation of methylated pararosanilines and demethylated derivatives of Michler\u27s ketone as the reaction intermediates was obtained for the photodecomposition of crystal violet in water solution. The mechanism of formation of the intermediates was discussed in terms of charges of individual atoms calculated using Spartan software. Photooxidation of crystal violet was shown to occur faster in hydrogen peroxide solution due to the higher oxidation potential of hydrogen peroxide and hydroxyl radicals. A mechanism similar to photooxidation in water solution was proposed since the formation of the same intermediates was observed. The semiconducting oxides Ti02 and ZnO were shown to be more efficient in crystal violet photodecomposition than any of the insulating oxides studied: SiO2, Al2O3, and MgO. Adsorption and wavelength dependent studies of titanium dioxide suspensions are consistent with a pathway in which a dye is oxidized by the surface-bound hydroxyl radicals formed by injection of an electron from adsorbed hydroxyl groups to photogenerated holes. Photogenerated electrons are subsequently scavenged by molecular oxygen to form O2- radical anions and ultimately hydroxyl radicals that can also oxidize the dye. Intermediates/products of photodecomposition were found to be adsorbed on the titanium dioxide surface, which could lower the catalytic efficiency of titanium dioxide as the reaction proceeds. Zinc oxide was found to be the most efficient oxide for crystal violet photodecomposition, but its low photochemical stability makes it unsuitable for industrial catalysis. Photochemically inert oxides were shown to influence the rate of crystal violet photodecomposition. Acceleration of photodecomposition in the presence of aluminum oxide can be explained by stabilization of the excited state of the dye by the aluminum oxide surface, and inhibition of photodecomposition in the presence of silicon dioxide can be explained by the formation of dye clusters on the silicon dioxide surface. Fast decolorization in magnesium oxide suspensions is due to the reaction of crystal violet with hydroxide ion to form a carbinol base. Titanium dioxide was concluded to be the best prospective catalyst for crystal violet photodecomposition of those studied, since it is highly efficient, relatively cheap and chemically stable

Radiation Pressure and Photon Momentum in Negative-Index Media

Radiation pressure and photon momentum in negative-index media are no different than their counterparts in ordinary (positive-index) materials. This is because the parameters responsible for these properties are the admittance, sqrt(epsilon/mu), and the group refractive index n_g of the material (both positive entities), and not the phase refractive index, n=sqrt(epsilon*mu), which is negative in negative-index media. One approach to investigating the exchange of momentum between electromagnetic waves and material media is via the Doppler shift phenomenon. In this paper we use the Doppler shift to arrive at an expression for the radiation pressure on a mirror submerged in a negative-index medium. In preparation for the analysis, we investigate the phenomenon of Doppler shift in various settings, and show the conditions under which a so-called "inverse" Doppler shift could occur. We also argue that a recent observation of the inverse Doppler shift upon reflection from a negative-index medium cannot be correct, because it violates the conservation laws.Comment: 14 pages, 8 figures, 17 equations, 5 reference

Theoretical analysis of the force on the end face of a nano-filament exerted by an outgoing light pulse

The slight deformations observed upon transmission of a light pulse through a short length of a silica glass nano-filament offer the possibility of determining the momentum of light inside the filament. Using precise numerical calculations that take into account not only the electromagnetic momentum inside and outside the filament, but also the Lorentz force exerted by a light pulse in its entire path through the nano-waveguide, we conclude that the net effect of a short pulse exiting the nano-filament should be a pull force on the end face of the filament.Comment: 11 pages, 7 figures, 13 equations, 7 reference